1. Field of the Invention
The present invention relates to a method and apparatus for acquiring binary data and clock signals from asynchronous input signals, such as from a magnetic tape storage device.
2. The Prior Art
Successful transmission from a data transmitter to a data receiver requires a sychronized clock signal. The clock signal may be disturbed by the fact that the incoming stream of data signals is incompletely received, as the result of variable transmission parameters, and disturbances either at the receiving location or along the transmission path. For example, when data is read from a magnetic tape storage device, the data is transmitted from the storage device without its own data clock, and it is therefore necessary, in order to retrieve the originally stored binary data, to generate a data clock from the data stream. This is done by a phase control circuit which must accomplish several different functions. In the case of frequency fluctuations in the data being received, such as changes in the sampling rate, the phase control circuit arrangement must be able to follow such frequency variations within a specific range and must therefore have the capability of locking into a variable frequency. Moreover, when one or more data pulses in the data stream is missing, the circuit arrangement must retain its previously determined frequency, i.e., exhibit holding behavior. Minor fluctuations in the timing of the data pulses, in the vicinity of their expected position, assuming a specific clock frequency, should not affect the determined clock frequency. In addition, the circuit arrangement must be able to resynchronize itself to the data stream as quickly as possible after an interruption in transmission.
Circuits which have been developed in the past for this purpose include analog as well as digital phase control circuits. Analog circuits require frequent readjustment and are therefore expensive, considering the time and expense required for down time and maintenence. Also, they are relatively unstable with the respect to their control function.
Digital phase control circuits or hybrid phase control circuits using mixtures of analog and digital circuits have been developed. These circuit arrangements typically include a phase detector and a controlled oscillator. Based on an internal clock generator, the oscillator is set to a nominal value, and the actual value of the momentary frequency and phase of the data clock of the received data signals is identified, by using a phase detector. When fluctuations are recognized, a new rated value is identified under given condition from a comparison of the detector conditions, and the oscillator is set to the new calculation frequency value. Such an arrangement is illustrated in U.S. Pat. No. 4,109,236. This arrangement operates in connection with a window, or a time period in which input pulses are expected, and the time position of such window is continuously adjusted in accordance with the frequency which is derived from the data already received. The received data signals are evaluation with an internal clock, and with the assistance of a main counter, the number of internal clock pulses between successive flux changes of the incoming data signals is averaged over a period of time. The mean value thus obtained furnishes an updated parameter for redefining the position of the window.
U.S. Pat. No. 4,357,707 shows a digital phase control circuit having a window with special characteristics. For example, the width of the window is variable, and its position and duration are both derived from the preceding two significant flux changes with respect to the windows in which they were detected.
In both of the phase control circuits referred to above, only digital components are used. Both the circuits proceed from the formulation of a mean value, based on evaluation of the length of preceding data signal periods, in order to eliminate brief duration fluctuations in the scanning or sampling rate, so as not to over compensate the phase control circuit for brief variations.
The fundamental task of a phase control circuit is to derive a manipulated variable by comparing incoming data to a rated value, in order to match a new rated value to the current actual value. From this point of view, the two phase control circuits referred to above both employ a new rated value determined by a hard-wired circuit which directly corresponds to a prior actual value averaged in a prescribed fashion. This calculation of mean value is determined only over an extended time period, which eliminates dependency of the function on brief duration fluctuations, but cannot respond well to greater phase modifications. A discontinuity in the phase which exceeds the capacity of the prior systems to track can lead to discontinous operation.
It is desirable not only to avoid the disadvantages of the analog or hybrid circuit arrangements but also fully to exploit the properties and possibilities of digital technology in generating the control function. In digital technology, appropriate means is available for the realization of the required complex functions, such as program controls or sequential combinational logical systems. However, when such means are employed, the complexity of the system increases, and the time required for executing the desired functions is also increased. It is therefore desirable to provide an arrangement which overcomes these difficulties.